The public interest and necessity of green energy vehicles has increase recently. Among these green energy vehicles, a fuel cell vehicle obtains driving force from a fuel cell.
FIG. 1 illustrates an exemplary fuel cell system mounted in an exemplary fuel cell vehicle in the arts.
As shown in FIG. 1, the fuel cell system 10 includes a fuel cell 11, a hydrogen tank 12, a hydrogen supply unit 13 for supply hydrogen to the fuel cell 11, an air supply unit 14 for supplying external air to the fuel cell 11, a Purge valve 15 for exhausting gases such as nitrogen and steam to the exterior, and a fuel cell control unit 16 for controlling operations of the fuel cell system 10.
The fuel cell 11 includes a hydrogen electrode 11a and an air electrode 11b, and the two electrodes 11a and 11b are separated by an electrolyte membrane 11c. Hydrogen supplied to the hydrogen electrode 11a reacts with oxygen in air supplied to the air electrode 11b, thereby generating electric energy. The electric energy is used as driving energy of the vehicle.
When the fuel cell system 10 is in a shut-off state, the composition of gases in the fuel cell 11 is changed depending on a shut-off duration. The graph in FIG. 2 illustrates a change in composition of gases depending on a shut-off duration. As shown in FIG. 2, the horizontal axis refers to the shut-off duration of the fuel cell system, and the vertical axis refers to the concentration of gases based on the shut-off duration. The shut-off duration of the fuel cell system 10 is divided into an initial shut-off duration (0 to t1), a middle shut-off duration (t1 to t2), and a latter shut-off duration (after t2).
In the initial shut-off duration, hydrogen remaining in the fuel cell 11 disappears by reacting with oxygen remaining in system piping. Accordingly, the concentration of hydrogen in the fuel cell 11 is gradually decreased, but the concentration of nitrogen is gradually increased.
In the middle shut-off duration, hydrogen and oxygen do not exist in the fuel cell 11, but the concentration of nitrogen is saturated.
In the latter shut-off duration, external air is flowed in the fuel cell 11 through fine cracks existing in system components such as valves or piping, such that the concentration of oxygen in the fuel cell 11 gradually increases. Accordingly, the concentration of nitrogen gradually decreases.
Although the composition of gases in the fuel cell is changed depending on the shut-off duration as described above, in the related arts, ignition in the typical fuel cell system may be performed without considering a change in composition of gases in a fuel cell.
According to the related art ignition method, for example, when the ignition is performed in a state in which the concentration of hydrogen is low, the fuel cell may be deteriorated due to a local lack of hydrogen. On the contrary, when the ignition is performed in a state in which the concentration of hydrogen is excessively high, the fuel efficiency and safety of the vehicle may be lowered due to excessive exhaustion of hydrogen.
In addition, according to the related art ignition method, when the shut-off duration belonging to the latter shut-off duration, a carbon catalyst of the air electrode may be corroded due to the influence of oxygen flowed in the fuel cell before the ignition. As such, the durability of the fuel cell may be degraded.